The present invention relates to a control valve for a continuously variable belt-drive automatic transmission for a motor vehicle, and more particularly to a transmission ratio control valve for controlling upshifting and downshifting of the transmission.
A known control system for a continuously variable belt-drive transmission comprises an endless belt running over a drive pulley and a driven pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid operated servo device so as to vary the running diameter of the belt on the pulleys in dependency on driving conditions. The system is provided with a hydraulic circuit including a pump for supplying oil to the servo devices, a line pressure control valve and a transmission ratio control valve. Each valve comprises a spool to control the oil supplied to the servo devices.
The transmission ratio control valve operates to upshift and downshift the transmission in accordance with the opening degree of a throttle valve of an engine and the speed of the engine. The line pressure control valve is adapted to control the line pressure in accordance with the transmission ratio and the engine speed. The line pressure is controlled to prevent the belt from slipping on the pulleys in order to transmit the output of the engine.
The spool in the transmission ratio control valve is shifted in accordance with a relationship between spring force responsive to a depression of an accelerator pedal of the vehicle and pitot pressure responsive to the engine speed. The spool is shifted to an oil supply position for upshifting the transmission and an oil drain position for downshifting the transmission, and stopped at a position where the spring force and the pitot pressure balance with each other.
The flow rate of supply oil or drain oil is decided in accordance with changing rate of an opening area of a supply port or a drain port, so that the transmission ratio changing speed at upshifting or downshifting is determined.
If the transmission ratio changing rate is too high for the control of the transmission ratio, overshooting of the control operation will occur because of the delay of the control operation. On the other hand, if the transmission ratio changing rate is slow, the shifting operation of the transmission is delayed.
Further, the pressure in the chamber of the drive pulley is set to have a level, for example, half of that of the line pressure. Thus, the conditions for supplying a high line pressure at upshifting are different from the conditions for draining a low pressure at downshifting. Such a difference between the conditions also affects the transmission ratio changing rate.
German Patent Application Laid-Open DE-OS 3607270 discloses a control valve in which a spool mounted in a valve body has a land corresponding to ports which are communicated with a chamber of a drive pulley for controlling the amount of supply or drain oil. The land has tapered portions formed on opposite circumferential edge portions thereof, thereby moderately changing the opening degree of the port.
However, in such a structure, the spool becomes longer by the length of the tapered portions compared with a spool without a tapered portion. Further, in order to reduce the changing rate of opening area, the tapered portion must be formed at a very small angle. Therefore, it is difficult to manufacture tapered portions with accuracy. In addition, the space between the tapered portion and the inside wall of the valve body is liable to be clogged with dust and chips.